The present invention relates to a mark for position detection and a mark detecting method and apparatus, and also relates to an exposure system. More particularly, the present invention relates to a mark for position detection formed on a plane surface of a substrate and a method and apparatus for detecting the mark, and also relates to an exposure system having the mark detecting apparatus as a device for detecting the position of the substrate. The mark for position detection according to the present invention is suitable for use in an exposure system to align a mask pattern and a photosensitive substrate in a photolithography process in which the substrate is exposed in accordance with the mask pattern to produce, for example, a semiconductor device.
In photolithography processes for producing micro devices, e.g. semiconductor devices, liquid-crystal display devices, image pickup devices (CCDs), or thin-film magnetic heads, a projection exposure system is used in which an image of a photomask or a reticle (hereinafter generally referred to as xe2x80x9creticlexe2x80x9d), which has a transfer pattern formed thereon, is projected onto a substrate coated with a photosensitive material (photoresist), e.g. a wafer or a glass plate (hereinafter referred to as xe2x80x9cwaferxe2x80x9d), through a projection optical system.
In this type of projection exposure system, alignment of the reticle and the wafer must be carried out with high accuracy before exposure. To perform the alignment, the wafer has marks for position detection (alignment marks) formed (transferred by exposure) in a preceding photolithography process. By detecting the position of such an alignment mark, the position of the wafer (or a circuit pattern on the wafer) can be accurately detected.
The alignment marks on the wafer are entirely unnecessary for the operating characteristics of the completed micro device. Therefore, it is desirable that the size of the marks be as small as possible. Alignment marks are generally set in the boundary regions between micro devices, which are known as xe2x80x9cstreet linesxe2x80x9d, i.e. xe2x80x9cmarginsxe2x80x9d for cutting the micro devices from each other after the completion of various processes. The street lines are belt-shaped regions each having a width of the order of from 70 to 90 micrometers. Therefore, the length of each shorter side of alignment marks is desirably not greater than 70 micrometers.
Some methods of detecting the position of a mark on a wafer have already been put to practical use. The mainstream of recent mark position detecting methods is an image detection method in which an optical mark image is detected, and the mark position is detected on the basis of the image intensity distribution.
Because the above-described alignment requires an extremely high degree of accuracy, the mainstream of conventional mark position detecting methods is such that marks (X mark and Y mark) are used exclusively for two orthogonal directions (X direction and Y direction), and the positions of these marks are successively measured. In general, line-and-space patterns having periodicity in the measuring directions have heretofore been used as the X and Y marks (marks for one-dimensional measurement).
FIGS. 8A and 8B show examples of the conventional marks for one-dimensional measurement. FIG. 8A shows a mark MX used for detection in the X direction. FIG. 8B shows a mark MY for detection in the Y direction. These marks MX and MY are used in a pair (there has been no specific restriction on the positional relationship between the two marks). In the conventional practice, position detection is first carried out with respect to one direction (X direction or Y direction) using one mark, and then position detection is carried out with respect to the other direction (Y direction or X direction) using the other mark.
As has been stated above, the length of each shorter side of these marks is demanded to be not greater than the street line width (in general, from 70 micrometers to 90 micrometers). Therefore, the widths of these marks in the non-detecting direction (Y direction for MX; X direction for MY) have generally been restricted to a length not greater than about 70 micrometers from the above restriction.
A mark detecting optical system for detecting the position of a mark as described above needs to be corrected for aberrations to an extremely high degree. The aberration correction includes not only the correction of aberrations due to errors in designing the optical system but also the correction of aberrations due to errors in machining, i.e. decentration of the lens and an error in the surface accuracy. In particular, errors in machining are difficult to eliminate completely. Therefore, after the assembly of the optical system, adjustment is made to minimize the residual aberrations at a specific xe2x80x9cmark detecting positionxe2x80x9d (one point in the detecting area near the optical axis of the detecting optical system), thereby reducing the influence of the residual aberrations during the detection. Accordingly, if the position for detecting a mark deviates from the above-described minimal aberration point, a detection error due to the residual aberrations arises, and the mark detection accuracy degrades.
In the above-described conventional technique, the mark detection for position detection is carried out with respect to the X and Y directions separately from each other. Therefore, it takes a long time to detect the marks, and this causes the processing capacity (throughput) of the projection exposure system to be reduced unfavorably.
In view of the above-described circumstances, a conventional technique uses two-dimensional marks that enable simultaneous detection with respect to both the X and Y two-dimensional directions.
FIG. 9 shows one example of two-dimensional marks for detection in both the X and Y directions. A mark MG shown in FIG. 9 per se has periodicity in two-dimensional directions. However, as will be clear from FIG. 9, the size of the mark edge (boundary between black and white), which is effective for the position detection in each of the X and Y directions, relative to the mark area undesirably reduces to approximately a half of that of one-dimensional marks (MX and MY) because of the periodicity in the two-dimensional directions. Therefore, the mark area must be increased in order to obtain a detection accuracy equal to that in the case of the one-dimensional marks (MX and MY). However, if the mark area is increased, the length of one side (or shorter side) of the mark becomes greater than the street line width (e.g. 100 micrometers or more in the case of FIG. 9), and hence a part of the mark undesirably extends over the circuit pattern on the wafer. Accordingly, the restriction on the mark formation position increases unfavorably.
If a two-dimensional mark Mt as shown in FIG. 10 is used in which an X-direction one-dimensional detection mark portion Ma and a Y-direction one-dimensional detection mark portion Mb are disposed in a side-by-side relation to each other, the length of each shorter side of the mark can be made not greater than the street line width.
However, the mark Mt shown in FIG. 10 suffers from problems in terms of the detection accuracy. That is, the mark detecting optical system has been adjusted such that the residual aberrations are minimized at a specific xe2x80x9cmark detecting positionxe2x80x9d (one point in the detecting area near the optical axis of the detecting optical system), as stated above. Therefore, if a detection mark portion for one direction, e.g. the Y-direction detection mark portion Mb (or the X-direction detection mark portion Ma), is disposed near the optical axis, the other mark portion Ma (or the mark portion Mb) for the other direction lies apart from the optical axis. Consequently, the detected value for the position of the latter mark portion Ma (or Mb) is adversely affected by the residual aberrations of the optical system when the position of the mark Mt is detected with respect to both the X and Y directions simultaneously. Consequently, detection errors increase unfavorably. (This problem will be described later in more detail to compare the present invention with the conventional technique in the description of the embodiments).
In view of the above-described circumstances, one object of the present invention is to provide a mark for position detection that enables the time required for mark detection to be shortened and makes it possible to effect position detection of high accuracy substantially independently of the residual aberrations of a detecting optical system.
Another object of the present invention is to provide an alignment mark capable of meeting a demand for the mark size.
Another object of the present invention is to provide a mark detecting method that enables the time required for mark detection to be shortened and that permits mark position detection to be effected with high accuracy substantially independently of the residual aberrations of a detecting optical system.
Another object of the present invention is to provide a mark detecting apparatus that enables the time required for mark detection to be shortened and that permits mark position detection to be effected with high accuracy substantially independently of the residual aberrations of a detecting optical system.
Another object of the present invention is to provide an exposure system capable of achieving an improvement in the throughput and of realizing registration of high accuracy.
The present invention provides a mark for position detection arranged as follows: The mark is formed on a substrate to detect the position of the substrate in a predetermined first axis direction (e.g. a Y-axis direction) and in a second axis direction (e.g. an X-axis direction) perpendicular to the first axis direction. The mark has a first pattern disposed near the center of the mark and having periodicity in the first axis direction, and second patterns respectively disposed near both sides of the first pattern in the second axis direction and each having periodicity in the second axis direction.
With the above-described arrangement, the first pattern is detected by a detecting optical system in a state where the detection center of the optical system is coincident with a predetermined reference point in a mark region in which the first pattern is formed (e.g. the center of the first pattern). That is, the first pattern is detected at the minimal aberration point of the detecting optical system. At the same time, the second patterns are detected at respective points symmetric with respect to the minimal aberration point, and the detected values for the positions of the second patterns are averaged. Thus, the position in the first and second axis directions of the mark for position detection can be detected with high accuracy substantially independently of the residual aberrations of the detecting optical system. Accordingly, the time required for mark detection can be shortened, and moreover, position detection of high accuracy can be effected substantially independently of the residual aberrations of the detecting optical system.
In this case, the period of each of the first and second patterns is desirably in the range of from about 6 micrometers to about 16 micrometers. The reason for this is as follows: The mark for position detection is generally formed on the substrate as a step-shaped mark by a photolithography process. Therefore, if the pattern period is smaller than 6 micrometers, the mark may be undesirably buried by the mark forming process. If the pattern period is larger than 16 micrometers, the number of pattern elements of the mark that can be captured on the image pickup area of an image pickup device (CCD), which is generally used for the mark detection, becomes excessively small, and the detection accuracy is degraded. Accordingly, if the pattern period is set in the range of from about 6 micrometers to about 16 micrometers, there are no such problems, and mark position detection of higher accuracy can be realized.
In these cases, the length of each short side of a mark region in which the first and second patterns are formed is desirably in the range of from about 50 micrometers to about 70 micrometers. The reason for this is as follows: As stated above, the mark for position detection is generally set in a belt-shaped boundary region (street line) between micro devices, which has a width of the order of 70 to 90 micrometers. Considering that a dicing saw used to cut the wafer along the street line has a width of the order of 70 micrometers at maximum, the length of each short side of the mark region is desirably set at a value not greater than 70 micrometers. By doing so, the mark for position detection can readily be formed within the street line. Therefore, the mark is not substantially restricted by the mark formation position. If the length of each short side of the mark region is smaller than 50 micrometers, the area of the mark becomes excessively small, causing the position detection accuracy to be degraded.
Therefore, the length of each short side of the mark region is preferably not less than 50 micrometers. Thus, in a case where the length of each short side of the mark region is set in the range of from about 50 micrometers to 70 micrometers, it is possible to meet a demand for the mark size satisfactorily.
In addition, the present invention provides a mark detecting method for detecting the position of the above-described mark for position detection in a predetermined first axis direction (e.g. a Y-axis direction) and in a second axis direction (e.g. an X-axis direction) perpendicular to the first axis direction. The method is characterized in that the position in the first axis direction of the mark is detected from a mark region near a predetermined detection center, and the position in the second axis direction of the mark is detected from each of two mark regions a predetermined distance away from both sides of the detection center in the second axis direction.
In addition, the present invention provides a mark detecting apparatus for detecting the position of the above-described mark for position detection formed on a substrate in a predetermined first axis direction (e.g. a Y-axis direction) and in a second axis direction (e.g. an X-axis direction) perpendicular to the first axis direction. The apparatus has a substrate stage on which the substrate formed with the above-described mark for position detection is placed, and which is movable in a reference plane, together with the substrate placed thereon; an image processing type mark detecting system that photoelectrically detects the mark; and an image processor that obtains the position of the mark in the first and second axis directions by processing a detection signal detected by the mark detecting system.
By virtue of the above arrangement, the substrate formed with the above-described mark for position detection is placed on the substrate stage; therefore, the mark can be photoelectrically detected by the mark detecting system in a state where the substrate stage is stationary. When the mark is photoelectrically detected by the mark detecting system, the image processor processes the detection signal detected by the mark detecting system, thereby obtaining the position in the first and second axis directions of the mark. Thus, it is possible to obtain the position of the mark in the two-dimensional directions in a state where the substrate stage is stationary, that is, by a single detecting operation. To detect the mark by the mark detecting system, the detection center of the detecting optical system, which constitutes the mark detecting system, is made coincident with the center of the first pattern, which constitutes the mark for position detection. Thus, the first pattern is detected at the minimal aberration point of the detecting optical system. Moreover, the second patterns are detected at two points symmetric with respect to the minimal aberration point, and the detected values for the positions of the second patterns are averaged. Thus, the position of the mark in the first and second axis directions can be detected with high accuracy substantially independently of the residual aberrations of the detecting optical system.
In addition, the present invention provides an exposure system in which an image of a pattern formed on a ask is projected by exposure onto a substrate coated with a photosensitive material through a projection optical system. The system has the above-described mark detecting apparatus as a device for detecting the position of the substrate.
The exposure system makes it possible to shorten the time required to detect the position detection mark on the substrate in comparison to the conventional practice that uses one-dimensional marks. Moreover, it is possible to detect the position of the mark in the first and second axis directions with high accuracy substantially independently of the residual aberrations of the detecting optical system, which constitutes the mark detecting system. Consequently, it becomes possible to achieve an improvement in the throughput and an improvement in the alignment accuracy and hence possible to improve the overlay accuracy.